Different techniques have been developed to identify or measure the color of different objects, such as paper, paint, or plastic. These color measurement techniques typically attempt to provide objective color measurements rather than subjective color measurements. However, conventional color measurement techniques are often poorly suited for measuring or identifying the color of a fluorescent material. A fluorescent material typically represents a material having a luminescence caused by absorption of radiation at one wavelength followed by re-radiation (often at a different wavelength) that ceases when the radiation stops. Conventional color measurement techniques are also often poorly suited for measuring or identifying the color of a phosphorescent material. Phosphorescence is a form of fluorescence where the re-radiation of light energy absorbed at one instant occurs over an extended time rather than at essentially the same instant.
Conventional color measurement techniques typically have difficulty measuring the color of a fluorescent or phosphorescent material for several reasons. One reason is that conventional color measurement techniques are usually based on or descended from techniques for measuring the color of non-fluorescent materials. Non-fluorescent materials usually have a radiance factor that is independent of illumination. In contrast, the radiance factor of a fluorescent or phosphorescent material is often strongly dependent on the spectral power distribution of illumination. In other words, the radiance factor of a fluorescent or phosphorescent material typically varies depending on the light shining on the fluorescent or phosphorescent material during the color measurement.
Conventional color measurement techniques often can accurately characterize the color of a fluorescent or phosphorescent material only for one or several specific illumination conditions. From these measurements, it is usually not possible to accurately predict the color of the fluorescent or phosphorescent material under illumination that differs significantly from the illumination used during the color measurement. This often leads to several problems. For example, there may be large disagreements between manufacturers and customers as to whether a particular object (such as a custom product) satisfies a color specification for that object. Also, this may lead to severe metamerism, where fluorescent or phosphorescent materials that appear substantially identical in color to one color measurement instrument appear substantially different in color to another color measurement instrument. In addition, color measurements made using the conventional color measurement techniques often provide inadequate or misleading information for modeling a coloring process. This often makes it difficult to implement quality control mechanisms for the coloring process and leads to poor quality control performance.
One prior color measurement technique for measuring the color of a fluorescent material involves producing a beam of light having a spectral distribution that varies over time. The beam of light is used to illuminate a material, and spectral power measurements are taken at different times. However, this color measurement technique may require a significant amount of time to work properly. Moreover, when measuring the property of a material that is moving relative to a color measurement instrument, the measurements are typically reliable only if the property does not vary over the distance moved while the measurements are made. For example, in a paper-making machine, a sheet of paper could move at up to 30 meters per second. During this time, only one or two reliable measurements might be formed, and those measurements may be unreliable if the material's property varies over shorter distances.
One prior technique for measuring the color of a phosphorescent material involves continuously illuminating an area of the material with light that spectrally matches the intended illumination in which the material will be used. In this case, light from the sample may include a phosphorescent component as well as fluorescent and reflected or transmitted components. Alternatively, the illumination of the material may be interrupted while the measurement of light from the material continues, allowing the phosphorescence alone to be measured and its variation with time ascertained.
These techniques suffer from the same failings as the conventional measurements of fluorescence, namely that the measurements are not indicative of the color of a material under illumination conditions different from those used for the measurements. Also, if the material exhibits phosphorescence instead of or in addition to fluorescence, the technique in which the spectral distribution of the illumination varies over time may produce incorrect measurements. This is because phosphorescence resulting from illumination at any instant affects the measured light for a significant time after that instant, while the conventional measurement techniques often presume that the measured light varies only in response to the simultaneous variation in illumination. As a result, the time-varying effects on the measurements caused by phosphorescence are conflated with the illumination-varying effects caused by fluorescence. The two effects typically cannot be distinguished, and neither can be reliably quantified from the measurements.